The present invention relates to power converters, and more particularly to closed-loop regulation of resonant power converters for maintaining operation in a minimal loss region.
Class E resonant power converters offer high efficiency and consequently, in operation, Class E resonant power converters are characterized by low power dissipation, low junction temperature and high reliability. Also, Class E resonant power converters are characterized by low sensitivity to component tolerances, enhancing manufacturing reproducibility and lessening manufacturing tolerances.
However, despite its extremely high efficiency, the Class E-type power converter has not gained wide acceptance in power converter applications because of the difficulties in regulating and control of the power converter. The Class E power converter must be operated in the "lossless region". Otherwise, large amounts of power are dissipated in the transistor switching devices of the power converter, resulting in damage to these components. To maintain operation in the "lossless region", it is necessary to maintain substantially zero voltage and zero slope conditions at the switch-on times for the power switching device. These conditions are disclosed in U.S. Pat. No. 3,919,656 of Sokal et al., for example. In known Class E converters, operation in the "lossless region" is established by selection of values for elements of the load and the load matching network. That is, the power converter includes a tuned network which establishes the operating frequency for the circuit. Consequently, variations in the load will result in a change in operating frequency, shifting the power converter operating point out of the "lossless region". Adjustment of duty cycle and the use of special switching devices in the Class E converters have been proposed, but have not provided entirely satisfactory results.
Another consideration is that in conventional Class E converters, regenerative feedback, or an independent drive oscillator, is used to drive the switching transistors. This renders the Class E circuit extremely sensitive to frequency variation because the "lossless region" of operation can only be maintained at one specific frequency for a specific load network. For even minor deviations from the Class E frequency, the switching losses become excessive and device destruction is likely. Maintaining Class E operation under conditions of changing inductance, or inductance and resistance, is essential to optimal circuit performance.